This invention relates to a fiber optics based radiation or neutron detector in which the fluorescence emitted from a phosphor or scintillator is guided not direct through a wavelength shifting fiber but first allowed to be incident on a partially scraped lateral side of a clear optical fiber and then guided to both ends of a wavelength shifting fiber.
Conventional fiber optics based radiation and neutron detectors are a radiation image detector and a neutron image detector that use a phosphor or a scintillator in combination with a wavelength shifting fiber (Nucl. Instr. and Meth., A430 (1999) 311-320, Japanese Patent Application Hei 10-366679 and Japanese Patent Application 2000-259443). These detectors are characterized by using the crossed fiber readout method to obtain position information. As shown in FIG. 17, a phosphor sheet or a scintillator plate is sandwiched between wavelength shifting fiber bundles that are placed in crossed planes at right angles and the incident position of radiation is determined by a coincidence counting technique such as to detect a radiation image. Alternatively, as shown in FIG. 18, two bundles of wavelength shifting fibers, one for detecting shorter wavelengths of fluorescence and one for detecting longer wavelengths of fluorescence, are placed in crossed planes at right angles and are overlaid with a phosphor sheet or a scintillator plate and the incident position of radiation is determined by a coincidence counting technique such as to detect a radiation image.
The wavelength shifting fibers used in those detectors first absorb the fluorescence as emitted from the phosphor or scintillator, then convert the absorbed wavelength to a longer one for detecting the fluorescence. As a result of the conversion, the wavelength of the fluorescence becomes so long as to exceed the range of detection sensitivity of a photomultiplier tube commonly used as a photo detector. In particular, when the wavelength shifting fiber is used in a neutron detector, its sensitivity for gamma-rays has been the cause of gamma-ray background.
In order to increase the detection sensitivity of the above-described methods, a plurality of sensors each comprising a phosphor or scintillator and wavelength shifting fibers are placed one on another but then the number of wavelength shifting fibers is so much increased that the detecting configuration becomes complicated.
If wavelength shifting fibers or optical fibers are bent at the right angle or at an angle close to it, practical design considerations have made it unavoidable to bend the fiber with a radius of curvature no smaller than several centimeters. However, this has put a limit on the effort to fabricate a compact detector.
A further problem with the radiation image detector or neutron image detector that use a phosphor or a scintillator in combination with wavelength shifting fibers is that if the length of the wavelength shifting fibers is increased, the wavelength shifted fluorescence is absorbed as it is transmitted and a decreasing amount of the fluorescence will arrive at the detector. Hence, it has been impossible to use very long fibers. In addition, as already mentioned, the wavelength shifting fibers themselves have sensitivity for gamma-rays, so if they are incorporated in a neutron detector, the gamma-ray background is a problem and if they are used in a gamma-ray detector, there is produced a background to the desired gamma-ray image. In order to avoid these problems, it has been necessary to minimize the length of the wavelength shifting fibers.